A video terminal (sometimes called video display terminal, or VDT for short) was a terminal on which the characters being output were 'painted' on a CRT via a purely electronic process. It was thus roughly the same functionality as the earlier printing terminals, such as Teletypes, but instead of the output being printed on paper by a mechanical system, it was displayed on the screen. Like the earlier printing terminals, they were used to allow a user to do input/output to a computer; they usually communicated with the computer via asynchronous serial line.
The original chief advantage was speed, since the all-electronic mechanism could run at much higher speeds; the disadvantage was that only a limited amount of information could be displayed at one time. (This was particularly true of early ones, which could only display very limited amounts of text - some as small as 12 lines of 40 characters).
Very early model provided only the same motion controls as their printing predecessors - line feed, carriage return, etc. However, the ability to move the cursor to specific screen locations was soon added, which made possible a predecessor of WYSIWYG, to wit, screen editors such as EMACS.
A few had the capability to do graphics, such as lines, points, etc.